1. Field of the Invention
The present invention relates to a sliding part such as an adjusting shim used for a valve train mechanism of an internal combustion engine.
2. Description of the Related Art
In recent years, in the automotive field, from the perspective of saving energy, improvement in fuel consumption has become urgent. As a measure to achieve this, in parallel with lighter vehicle weight and improved engine heat efficiency, the reduction of power loss for engines and the like has become an important task. In particular, among the types of engine power loss, as a measure for reducing the power loss in the valve train mechanism, most studies have focused on lightening parts with the object of lightening inertial weight, and on reducing the friction work for sliding parts.
FIG. 3 shows a specific example of the valve train mechanism of an automobile engine. In FIG. 3, 1 is a cylinder head of the engine, 2 is a cam, 3 is a valve lifter, 4 is an adjusting shim, 5 is a suction and exhaust valve, 6 is a valve seat, and 7 is a valve spring. For the valve train mechanism shown in FIG. 3, by driving the valve lifter 3 through the cam 2, the displacement of the cam 2 is conveyed to the suction and exhaust valve 5.
For the aforementioned valve train mechanism, the adjusting shim 4 is placed between the cam 2 and the valve lifter 3, and therefore the adjusting shim 4 slides with the cam 2 and the valve lifter 3. This adjusting shim 4 is used for adjusting the valve clearance, and from the past was usually produced from metal. For this kind of adjusting shim 4 as well, there is a need for lighter weight and improved abrasion resistance to reduce power loss.
As a measures for reducing power loss for the aforementioned valve train mechanism, for example, Japanese patent publication 6-294307 discloses the use of diamond for the preparation of an adjusting shim as a sliding part, or depositing diamond on the base material of the adjusting shim in order to reduce friction work, in other words to reduce the coefficient of friction xcexc.
However, with this adjusting shim, it is necessary to finish the diamond surface to a smooth mirror surface, and because diamond is a very difficult material to cut, the processing cost becomes very expensive, and since diamond is a high cost material, the overall cost becomes very high. Also, when the base material is a metal, there is a big difference in the modulus of longitudinal elasticity of the metal and the diamond that covers it, so internal stress occurs at the interface of these two materials, leading to the problem of the diamond peeling from the metal base material.
Taking into consideration these problems with the prior art, an object of the present invention is to provide a diamond-coated sliding part that is light weight, excellent abrasion resistance, and prevents abrasion of the material of the mating member while being effective in reducing the power loss.
To achieve the aforementioned object, the diamond-coated sliding part provided by the present invention is a sliding part comprising a base material made of silicon nitride or sialon having a diamond coating layer on the surface thereof, in which the profile bearing length ratio (tp) at a cutting level of 0.1 xcexcm specified in Japanese Industrial Standard (JIS) B 0601 is 60% or greater for the sliding surface of the diamond coating layer.
By performing a finishing processing on only a small part of peaks of diamond particles protruding from the surface of the diamond coating layer after the formation of the aforementioned diamond coating layer, or by controlling various conditions during the above film formation, the profile bearing length ratio (tp) is adjusted as noted above, and the diamond coating layer surface is made smooth or having no projecting parts.